tests: Pass argument to client main

Change the API to pass an "void *" argument to the client main
function, allowing the caller to call the same main function with
different input.

A helper (client_create_noarg) is added for when no argument is passed,
and the existing test cases are changed to use this function instead.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>